1. Field of the Invention
This invention relates generally to the field of spectroscopy. More specifically, the invention relates to a method and device for resonant optothermoacoustic (ROTA) spectroscopy.
2. Background of the Invention
Photoacoustic spectroscopy (PAS) is an analytical method that involves stimulating a sample with modulated light and detecting the resulting sound waves emanating from the sample. A photoacoustic measurement can be made as follows. Methods and devices related to detecting photoacoustic signals in a fluid are discussed at length in U.S. Pat. No. 7,245,380 to Kosterev, which is incorporated herein in its entirety. First, light is used to stimulate molecules within a sample. Such stimulation can include, for example, absorption of the light by the molecule to change an energy state of the molecule. As a result, the stimulated molecule enters an excited state. Optical excitation is followed by the energy transfer processes (relaxation) from the initially excited molecular energy level to other degrees of freedom, in particular translational motion of the fluid molecules. During such relaxation, heat, light, volume changes and other forms of energy can dissipate into the environment surrounding the molecule. Such forms of energy cause expansion or contraction of materials within the environment. As the materials expand or contract, sound waves are generated.
In order to produce identifiable sound waves, or photoacoustic signals, the light is pulsed or modulated at a specific resonant acoustic or modulation frequency f (having a modulation period 1/f ), sometimes also described by the cyclic frequency Ω=2πf. The sample environment may be enclosed and may be constructed to resonate at the modulation frequency. An acoustic detector mounted in acoustic communication with the sample environment can detect changes occurring as a result of the modulated light stimulation of the sample. Because the amount of absorbed energy is proportional to the concentration of the absorbing molecules, the acoustic signal can be used for concentration measurements.
In typical PAS, a resonant acoustic cavity or sample cell with a quality factor Q is used to isolate and amplify sound wave signals, thereby increasing sensitivity of detection. The light intensity or wavelength is modulated at f. The absorbed energy is accumulated in the acoustic mode of the sample cell during Q oscillation periods. Hence, the acoustic signal is proportional to the effective integration or energy accumulation time t, where t=Q/f. Most often the Q factor is in the range 40-200 and f=1,000-4,000 Hz. Thus, in a non-limiting example, Q may equal 70 and f=1250 Hz, with the result that t=0.056 s.